Light scanning device and thrust-direction force canceling method

ABSTRACT

At least a part of a first thrust-direction force generated by air resistance received by at least one surface of reflection surfaces arrayed in a rotating direction of a polygon mirror and tilted with respect to a rotation axis of the polygon mirror is canceled by a second thrust-direction force generated by air resistance received by a surface tilted with respect to the rotation axis in a direction opposite to the surface where the first thrust-direction force is generated.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Division of application Ser. No. 12/023,674 filedon Jan. 31, 2008 now U.S. Pat. No. 8,027,076, the entire contents ofwhich are incorporated herein by reference.

This application claims the benefit of U.S. Provisional Application No.60/887,502 filed Jan. 31, 2007, the entire contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light scanning technique ofreflecting and deflecting incident light by a rotary deflector, andparticularly a technique of reducing the influence of a thrust-directionforce generated by air resistance received by a reflection surface ofthe rotary deflector.

2. Description of the Related Art

Traditionally, there has been known a technique of causing a luminousflux from a light source to scan the photoconductive surface of each ofthe plural photoconductors by using a rotary deflector and thus formingan image such as an electrostatic latent image on the photoconductors.

In the traditional technique, plural reflection surfaces of the rotarydeflector which carries out scanning of the plural photoconductors witha light beam are tilted in the same direction with respect to therotation axis (see, for example, JP-A-2000-2846).

In the traditional technique, since the plural reflection surfaces ofthe rotary deflector are tilted in the same direction with respect tothe rotation axis, a deviated thrust-direction force directed toward oneof the directions of rotation axis (thrust directions) is generated bythe influence of air resistance when the rotary deflector rotates.

In such a traditional light scanning technique, the rotary deflector maybe driven at plural patterns of rotation speeds in order to deal withdifferent printing speeds, and the force applied in the axial directionis changed by the variance in the rotation speed. Such variance in thethrust-direction force causes change in the position in the axialdirection of the rotary deflector and hence may cause variance inoptical characteristics.

Also, in circumstances where the position in the direction of rotationaxis of the rotary deflector changes in this manner, it is necessary tosecure a broad effective reflection area in the axial direction of therotary deflector (to increase size of the rotary deflector in thedirection of rotation axis) in order to make a luminous flux incident inthe effective reflection area on the reflection surfaces of the rotarydeflector even when the change in the position has occurred.

Such expansion of the effective reflection range of the rotary deflectorincreases windage loss at the time when the rotary deflector rotates,and thus increases motor load and noise.

The change in the position in the direction of rotation axis of therotary deflector as described above can be restrained if a bearing orthe like that can deal with the thrust-direction force is employed.However, there is a problem that the device becomes expensive.

SUMMARY OF THE INVENTION

It is an object of an aspect of the invention to provide a technique ofreducing adverse effects of a thrust-direction force generated by airresistance received by reflection surfaces of a rotary deflector, in alight scanning technique to reflect and deflect incident light by therotary deflector.

A light scanning device according to an aspect of the inventionincludes: a rotary deflector configured to reflect and deflect anincident luminous flux by plural reflection surfaces arrayed in arotating direction and thus cause the incident luminous flux to scan apredetermined direction, in which at least one of the plural reflectionsurfaces is tilted with respect to the rotation axis of the rotarydeflector; and a thrust-direction force canceling unit supported in arotatable manner integrally with the plural reflection surfaces of therotary deflector, and having a wind receiving surface tilted withrespect to the rotation axis of the rotary deflector in a directionopposite to at least one surface of the reflection surfaces tilted withrespect to the rotation axis.

A light scanning device according to another aspect of the inventionincludes: a rotary deflector configured to reflect and deflect anincident luminous flux by plural reflection surfaces arrayed in arotating direction and thus cause the incident luminous flux to scan apredetermined direction, in which at least one of the plural reflectionsurfaces is tilted with respect to the rotation axis of the rotarydeflector; and thrust-direction force canceling member supported in arotatable manner integrally with the plural reflection surfaces of therotary deflector, and for generating, by air resistance, a force in adirection opposite to a thrust-direction force generated by airresistance received by at least one surface of the reflection surfacestilted with respect to the rotation axis.

An image forming apparatus according to still another aspect of theinvention includes: a light scanning device having a configuration asdescribed above; a photoconductor on which an electrostatic latent imageis formed by a luminous flux cast for scanning by the light scanningdevice; and a developing unit configured to develop the electrostaticlatent image formed on the photoconductor.

A thrust-direction force canceling method according to an aspect of theinvention includes canceling at least a part of a first thrust-directionforce generated by air resistance received by at least one surface ofreflection surfaces arrayed in a rotating direction of a rotarydeflector and tilted with respect to the rotation axis of the rotarydeflector, by a second thrust-direction force generated by airresistance received by a surface tilted with respect to the rotationaxis in a direction opposite to the surface where the firstthrust-direction force is generated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing an exemplary configuration of an image formingapparatus having a light scanning device according to the firstembodiment of the invention.

FIG. 2 is a plan view for explaining the details of the light scanningdevice 3 according to the embodiment.

FIG. 3 is a side view for explaining the configuration of the peripheryof the light scanning device 3 in the image forming apparatus 1according to the embodiment.

FIG. 4 is a view showing the planar shape of a polygon mirror accordingto the first embodiment of the invention and the shape of eachreflection surface as viewed from a lateral side.

FIG. 5 is a plan view showing an optical path in the light scanningdevice according to the first embodiment of the invention.

FIG. 6 is a longitudinal sectional view of the optical path in the lightscanning device according to the first embodiment of the invention, asenlarged in the sub scanning direction.

FIG. 7 is a view showing an exemplary coordinate system at the time ofdefining the shape of a lens surface.

FIG. 8 is a view showing an exemplary shape definition formula for alens surface.

FIG. 9 is a table showing optical layout data in the light scanningdevice according to the first embodiment of the invention.

FIG. 10 is a table showing paraxial characteristic data in the lightscanning device according to the first embodiment of the invention.

FIG. 11 is a table showing decentration and tilt data in the lightscanning device according to the first embodiment of the invention.

FIG. 12 is a table showing coefficient data representing lens shape andso on in the light scanning device according to the first embodiment ofthe invention.

FIG. 13 is a graph showing the relation between stray light and positionin the main scanning direction of a ray in the light scanning deviceaccording to the first embodiment of the invention.

FIG. 14 is a plan view showing an optical path in a light scanningdevice according to the second embodiment of the invention.

FIG. 15 is a longitudinal sectional view of the optical path in thelight scanning device according to the second embodiment of theinvention, as enlarged in the sub scanning direction.

FIG. 16 is a table showing optical layout data in the light scanningdevice according to the second embodiment of the invention.

FIG. 17 is a table showing paraxial characteristic data in the lightscanning device according to the second embodiment of the invention.

FIG. 18 is a table showing decentration and tilt data in the lightscanning device according to the second embodiment of the invention.

FIG. 19 is a table showing coefficient data representing lens shape inthe light scanning device according to the second embodiment of theinvention.

FIG. 20 is plan view of the optical path of a ray cast for scanning bythe light scanning device according to the second embodiment of theinvention, as viewed from above.

FIG. 21 is a side view of the optical path of a ray cast for scanning inan image forming apparatus having the light scanning device according tothe second embodiment of the invention, as viewed from a lateral side.

FIG. 22 is a graph showing the relation between stray light and positionin the main scanning direction of a scanning ray in the light scanningdevice according to the second embodiment of the invention.

FIG. 23 is a plan view showing an optical path in a light scanningdevice according to the third embodiment of the invention.

FIG. 24 is a longitudinal sectional view of the optical path in thelight scanning device according to the third embodiment of theinvention, as enlarged in the sub scanning direction.

FIG. 25 is a table showing optical layout data in the light scanningdevice according to the third embodiment of the invention.

FIG. 26 is a table showing paraxial characteristic data in the lightscanning device according to the third embodiment of the invention.

FIG. 27 is a table showing decentration and tilt data in the lightscanning device according to the third embodiment of the invention.

FIG. 28 is a table showing coefficient data representing lens shape inthe light scanning device according to the third embodiment of theinvention.

FIG. 29 is plan view of an optical path in a light scanning deviceaccording to the fourth embodiment of the invention.

FIG. 30 is a longitudinal sectional view of the optical path in thelight scanning device according to the fourth embodiment of theinvention, as enlarged in the sub scanning direction.

FIG. 31 is a table showing optical layout data in the light scanningdevice according to the fourth embodiment of the invention.

FIG. 32 is a table showing paraxial characteristic data in the lightscanning device according to the fourth embodiment of the invention.

FIG. 33 is a table showing decentration and tilt data in the lightscanning device according to the fourth embodiment of the invention.

FIG. 34 is a table showing coefficient data representing lens shape inthe light scanning device according to the fourth embodiment of theinvention.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, embodiments of the invention will be described withreference to the drawings.

First Embodiment

First, the first embodiment of the invention will be described.

FIG. 1 is a view showing an exemplary configuration of an image formingapparatus having a light scanning device according to the embodiment ofthe invention. Here, an example is shown in which a color printerapparatus is used as the image forming apparatus according to thisembodiment. However, the image forming apparatus is not limited to this.As a matter of course, a color copying apparatus, a facsimile apparatusand so on may also be used.

The image forming apparatus 1 according to this embodiment has a lightscanning device 3 which generates scanning light corresponding to animage signal, and an image forming section 5 which makes visible anelectrostatic latent image formed on a photoconductor by the scanninglight from the light scanning device 3, by using toner, and transfersand outputs the formed toner image onto a paper P.

The paper P is held in a paper holding unit 7. The paper holding unit 7can hold predetermined number of sheet-like papers P and supplies thepapers P to the image forming section 5 according to the timing when atoner image is formed by the image forming section 5.

A carrying path 9 for carrying the papers P from the paper holding unit7 toward the image forming section 5 is provided between the paperholding unit 7 and the image forming section 5. The carrying path 9guides the papers P to a fixing device 11 which fixes the toner imagetransferred onto the paper P, to the paper P, via a transfer positionwhere the toner image formed by the image forming section 5 istransferred to the paper P.

The image forming section 5 has a transfer belt 13 formed by, forexample, shaping an insulating film with a predetermined thickness inthe shape of an endless belt. The material of the transfer belt 13 isnot limited to the insulating film. A thin metal sheet with its surfaceprotected by resin or the like may also be used.

The transfer belt 13 is wound on a driving roller 15 and a followingroller 19, and a predetermined tension is applied to the transfer belt13 by these rollers. As the driving roller 15 is rotationally driven, anarbitrary position on the transfer belt 13 parallel to the axial line ofthe driving roller 15 is moved in the direction of an arrow A. In otherwords, the belt surface of the transfer belt 13 is circulated in onedirection at a speed at which the outer circumferential surface of thedriving roller 15 is moved.

In a section where the belt surface of the transfer belt 13 is movedsubstantially flatly in the state where the predetermined tension isapplied, first, second, third and fourth image forming units 21, 22, 23and 24 are arrayed at predetermined intervals.

The first image forming unit 21, the second image forming unit 22, thethird image forming unit 23 and the fourth image forming unit 24 includedeveloping devices 21A, 22A, 23A and 24A which house toner of colors Y(yellow), M (magenta), C (cyan) and K (black), and photoconductive drums21B, 22B, 23B and 24B which hold an electrostatic image to be developedby each developing device.

On the surface (outer circumferential surface) of the photoconductivedrum of each of the image forming units 21 to 24, an electrostaticlatent image corresponding to an image of the color to be developed bythe developing device set in the image forming unit is formed by thescanning light from the light scanning device 3. The electrostaticlatent images thus formed on the photoconductors are developed with thetoner supplied from the corresponding developing devices.

The first to fourth image forming units 21 to 24 are facing transferrollers 31 to 34, with the transfer belt 13 provided between them. Thesetransfer rollers 31 to 34 push the side of the transfer belt 13 which isnot facing the photoconductors, to the photoconductive drums, and thustransfer the toner images on the photoconductive drums to the paper Pheld and carried on the transfer belt 13.

In the image forming apparatus 1 in which the developing devices 21A,22A, 23A and 24A, photoconductive drums 21B, 22B, 23B and 24B, thetransfer rollers 31, 32, 33 and 34 and the transfer belt 13 are arrayedas described above, image light generated as an image signal supplied byan image signal supply unit, not shown, is supplied to the lightscanning device 3 for each color component, is exposed to the surface ofthe photoconductive drums integrated with the developing devices whichhold toner of the corresponding colors.

At this time, in the individual image forming units 21 to 24,electrostatic latent images are formed on the photoconductive drums inpredetermined timing in such a manner that toner images to besequentially transferred are superimposed on each other on the paper P,and the electrostatic latent images are developed by the correspondingdeveloping devices.

The toner images formed on the photoconductive drums 213, 22B, 23B and24B of the individual image forming units 21 to 24 are transferred tothe paper P on the transfer belt 13 by the transfer rollers 31 to 34corresponding to the individual photoconductive drums 21B, 22B, 23B and24B. At this time, as the transfer belt 13 is moved at a predeterminedspeed, the toner images of Y, M, C and K are sequentially stacked on thepaper P on the transfer belt 13.

In this embodiment, an example where roller units are used as thetransfer rollers 31 to 34 is described, as shown in FIG. 1. However,voltage generators such as Scorotron may also be employed as transfermeans.

At a predetermined position in the carrying path 9, registration rollers61 are provided which temporarily stop the paper P guided from the paperholding unit 7 to the transfer position. Of the registration rollers 61,at least one roller rotates in a predetermined direction and the otherroller is pressed against the one roller by a predetermined pressure viaa press-contact mechanism, not shown.

As the paper P guided through the carrying path 9 from the paper holdingunit 7 toward the transfer position is butted against a nipping part ofthe registration rollers 61 in the stop state, its skew is corrected.

FIG. 2 is a plan view for explaining the details of the light scanningdevice 3 in this embodiment. FIG. 3 is a side view for explaining theconfiguration of the periphery of the light scanning device 3 in theimage forming apparatus 1 according to this embodiment.

The light scanning device 3 has the function of scanning thephotoconductive surfaces of the plural photoconductive drums 21B, 22B,23B and 24B in the main scanning direction with a luminous flux from alight source. Electrostatic latent images are formed on thephotoconductive surfaces of the photoconductive drums 21B, 22B, 23B and24B by the luminous flux cast for scanning by the light scanning device3. The electrostatic latent images formed on the photoconductive drumsare developed by the developing devices 21A, 22A, 23A and 24A withdevelopers of the colors corresponding to the individual photoconductivedrums.

A polygon mirror (rotary deflector) 35 reflects and deflects an incidentluminous flux by plural reflection surfaces 35Y, 35M, 35C and 35Karrayed in the rotating direction of the polygon mirror 35,corresponding to the plural photoconductive drums, and thus causes theincident luminous flux to scan the main scanning direction(predetermined direction). Also, the tilt angle of the plural reflectionsurfaces 35Y, 35M, 35C and 35K of the polygon mirror 35 to the rotationaxis 35 p of the polygon mirror 35 is set at an angle corresponding tothe photoconductor to which each reflection surface corresponds. Here,the reflection surfaces 35Y, 35M, 35C and 35K are supported in anintegrally rotatable manner about the rotation axis 35 p as the centralaxis of rotation. Here, the configuration in which all the reflectionsurfaces of the polygon mirror 35 are tilted with respect to therotation axis 35 p is described as an example. However, theconfiguration is not limited to this and the invention can be applied toany configuration in which at least one of these plural reflectionsurfaces is tilted with respect to the rotation axis 35 p.

A pre-deflection optical system 70 has an LD array 71 including four LDswhich are arranged at different positions from each other in the subscanning direction (direction of the rotation axis of the polygonmirror) orthogonal to the main scanning direction and can flashindependently of each other, a finite lens (or collimating lens) 72which converts divergent light from the LD array 71 into convergedlight, parallel light or moderate diffuse light, an aperture, not shown,and cylindrical lenses 74A and 743 which condense the luminous flux inthe vicinity of the polygon mirror 35.

With this configuration, the pre-deflection optical system 70 shapeslight from the LD array 71 into a luminous flux having a cross sectionwhich is long in the main scanning direction, then guides the luminousflux toward the polygon mirror 35, and condenses the luminous flux inthe sub scanning direction in the vicinity of the reflection surface ofthe polygon mirror 35.

A post-deflection optical system 37 includes plural optical elementsmade of a resin material such as plastics. Specifically, thepost-deflection optical system 37 has an fθ1 lens 37A having a free-formsurface with such power distribution that power continuously changes,and fθ2 lenses 37Y, 37M, 37C and 37K and cover glasses 38Y, 38M, 38C and38K provided corresponding to the individual photoconductive drums.

With this configuration, the post-deflection optical system guidesluminous fluxes reflected and deflected respectively by the pluralreflection surfaces 35Y to 35K of the polygon mirror 35, to thephotoconductive surfaces of the photoconductive drums 213, 22B, 23B and24B corresponding to the reflection surfaces through different opticalpaths from each other.

Specifically, the luminous fluxes reflected by the reflection surfaces35Y, 35M, 35C and 35K are guided to first folding mirrors 33Y, 33M, 33Cand 33K and are further folded by second folding mirrors 34Y, 34M, 34Cand 34K to reach the corresponding photoconductive drums. The luminousfluxes, thus folded, pass through the fθ2 lenses 37Y, 37M, 37C and 37K,then pass through the cover glasses 38Y, 38M, 38C and 38K, and areguided to the photoconductive drums. In this case, it is important thatthe components should be arranged in such a manner that each luminousflux and the optical element of each folding mirror or the like shouldnot interfere with each other.

Also, in this embodiment, since the polygon mirror 35 has eightreflection surfaces, if one luminous flux is made incident on thepolygon mirror, color information of the four colors can be written fortwo lines on each photoconductor as the polygon mirror 35 rotates by asingle turn. Here, a so-called “multi-beam optical system” is employedin which the LD array 71 emits four luminous fluxes. Therefore, colorinformation of the four colors can be written for eight lines on eachphotoconductor as the polygon mirror 35 rotates by a single turn.

The fθ1 lens 37A and the fθ2 lenses 37Y, 37M, 37C and 37K have theircurvatures changed independently in the two directions, that is, themain scanning direction and the sub scanning direction. The fθ1 lens 37Ain this case is equivalent to a shared optical element. The powerdistribution of the fθ1 lens 37A and each of the fθ2 lenses 37Y, 37M,37C and 37K is set in such a such manner that the luminous fluxreflected and deflected by the polygon mirror 35 so as to be guided toeach of the plural photoconductive drums 21B, 22B, 23B and 24B isprovided with power which causes the luminous flux guided to thephotoconductive surface by the post-deflection optical system 37 to havepredetermined optical characteristics (for example, characteristics thatsatisfy predetermined conditions with respect to the beam diameter ofthe luminous flux, the degree of curving of the scanning line, theposition of the luminous flux with respect to the scanning range, and soon) on the photoconductive surface according to the position ofincidence of the luminous flux. In this manner, the shared opticalelement has a smooth lens surface which acts on all the luminous fluxesreflected and deflected by the plural reflection surfaces of the polygonmirror 35.

In this manner, as a part of the optical elements which aretraditionally provided independently for each photoconductor iscollectively used as a shared optical element and all the luminousfluxes to be guided to the plural photoconductors are provided withpower by the shared optical element, it can contribute to reduction inthe arrangement space of the optical components to be arranged in thesub scanning direction. Also, since the number of optical components tobe arranged can be reduced, deterioration in optical characteristics dueto an arrangement error of each optical component and so on can beavoided and it can also contribute to reduction in cost.

Moreover, as a part of the optical elements provided independently foreach photoconductor is collectively used as a shared optical element,the tilt angle of each reflection surface of the polygon mirror can beset at a small angle and the arrangement space in the sub scanningdirection of the optical system can be reduced. Also, occurrence ofasymmetrical wavefront aberration which increases in the case where thereflection surfaces of the polygon mirror have a large tilt angle can berestrained, and therefore improvement in image forming characteristicscan be realized. Moreover, as the light scanning device having such aconfiguration is applied to an image forming apparatus, it is possibleto contribute to reduction in size of the image forming apparatus andstable image quality in image forming processing.

Here, “predetermined optical characteristics” refer to opticalcharacteristics which are desirable in order to form electrostaticlatent images on the photoconductive surfaces of the photoconductors. Asthe incident luminous flux from the pre-deflection optical system to thepolygon mirror is condensed in the vicinity of the reflection surfaces(a conjugate relation is formed in the sub scanning direction betweenthe reflection surfaces of the polygon mirror and the photoconductivesurfaces of the photoconductors), a shift of beam position in the subscanning direction due to the tilt of each reflection surface of thepolygon mirror is restrained (correction of face tangle error).

The polygon mirror 35 is fixed on the bearing surface on a rotor fixedto the shaft of a polygon motor 36. This polygon motor 36 isrotationally driven at a predetermined speed (number of rotations) fordeflection scanning. The number of the reflection surfaces provided onthe polygon mirror 35 and the number of rotations are prescribed inaccordance with an output request (that is, resolution, printing speedand the like required of the image forming apparatus 1).

FIG. 4 is a view showing the planar shape of the polygon mirroraccording to the first embodiment of the invention and the shape of eachreflection surface as viewed from a lateral side.

In FIG. 4, the side of a reference surface A represents the surfaceinstalled on a base part of the polygon motor 36. In FIG. 4, the surfacea and the surface e of the polygon mirror 35 are set at a tilt angle θ₁with respect to the rotation axis of the polygon mirror 35. The surfaced and the surface h are set at a tilt angle θ₂. The surface c and thesurface g are set at a tilt angle θ₃. The surface b and the surface fare set at a tilt angle θ₄.

Moreover, in this example, in terms of the absolute value of the tiltangle of a polygon mirror surface to the central axis of rotation 35 pof the polygon motor (central axis of the polygon mirror), θ₁ and θ₃ arethe largest and equal to each other with opposite signs. (That is, therelation of θ₁=−θ₃ holds). Here, a value of tilt angle θ with a “−”(negative) sign means that the surface tilts in a direction ofapproaching the direction of the rotation axis (central axis of thepolygon mirror) 35 p as it moves away from the reference surface A.

That is, in this embodiment, in the polygon mirror including reflectionsurfaces having a tilt with respect to the rotation axis of the polygonmirror, the plural reflection surfaces include the surfaces a and ehaving the maximum tilt angle with respect to the rotation axis and thereflection surfaces c and g having the same absolute value of angle fromthe rotation axis as the reflection surface a and having the oppositetilt direction (a pair of surfaces having the maximum absolute value oftilt angle is set at a tilt angle ±θ₁). In this case, one of thereflection surfaces a and e and one of the reflection surfaces c and gserve as wind receiving surfaces to cancel or reduce a thrust-directionforce generated by air resistance received by the other surfaces. Also,as the tilt angle of the pair of surfaces having the maximum absolutevalue of tilt angle is set at ±θ₁, occurrence of asymmetrical wavefrontaberration which increases in the case where the reflection surfaces ofthe polygon mirror have a large tilt angle can be restrained, and henceimprovement in image forming characteristics can be realized.

In this embodiment, as an example, the thrust-direction force generatedby air resistance received by the reflection surface tilted toward oneside with respect to the rotation axis of the polygon mirror is almostperfectly canceled by the thrust-direction force generated by airresistance received by the reflection surface tilted toward the otherside with respect to the rotation axis. However, the way of cancelingthe thrust-direction force is not limited to this. For example, if itsuffices to cancel only a thrust-direction force generated by a part ofthe plural reflection surfaces tilted toward one side with respect tothe rotation axis of the polygon mirror (if slight deviation of thethrust-direction force can be tolerated), a reflection surface that cangenerate a thrust-direction force in the opposite direction capable ofcanceling the thrust-direction force generated by the part of thereflection surfaces (a wind receiving surface tilted in the oppositedirection to the part of the reflection surfaces with respect to therotation axis) can be provided. Here, the reflection surface which formsthis wind receiving surface serves to reflect and deflect a luminousflux and also serves as a thrust-direction force canceling unit.

Also, a second thrust-direction force to cancel a first thrust-directionforce generated by air resistance received by a certain reflectionsurface need not necessarily be generated by the same number ofreflection surfaces as the former reflection surface(s), having the sameabsolute value of tilt angle. That is, it suffices to generate thesecond thrust-direction force that can cancel at least a part of thefirst thrust-direction force, as a result of air resistance received bya reflection surface which serves as a wind receiving surface. Forexample, the first thrust-direction force generated by a reflectionsurface having a certain tilt angle can be canceled or reduced by thesecond thrust-direction force generated by plural reflection surfacestilted in the opposite direction to the former reflection surface andhaving a broader tilt angle than that reflection surface. In thismanner, for example, the second thrust-direction force can be generatedby a greater (or smaller) number of reflection surfaces than the numberof reflection surfaces that generate the first thrust-direction force tobe canceled or reduced.

In the case where only a part of the plural reflection surfaces of thepolygon mirror is tilted in the opposite directions, it is preferablethat the surfaces having the largest tilt angle with respect to therotation axis should form a pair. Generally, the surfaces having thelargest tilt angle with respect to the rotation axis generate thelargest thrust-direction force when the polygon mirror rotates.Therefore, as the thrust-direction force is canceled or reduced, theabsolute value of the thrust-direction force applied to the polygonmirror at the time of rotation can be reduced.

In this manner, in the light scanning device according to thisembodiment, at least a part of the first thrust-direction forcegenerated by air resistance received by at least one surface (forexample, the reflection surface a) of the reflection surfaces arrayed inthe rotating direction of the polygon mirror 35 and tilted with respectto the rotation axis 35 p of the polygon mirror 35 is canceled by thesecond thrust-direction force generated by air resistance received by asurface (for example, the reflection surface c) tilted with respect tothe rotation axis 35 p in the opposite direction to the surface wherethe first thrust-direction force is generated (thrust-direction forcecanceling method).

As the polygon mirror of such a configuration is employed, shift of thepolygon mirror in the axial direction can be restrained which is causedby the generation of a deviated thrust-direction force due to theinfluence of air resistance received by the reflection surfaces when thepolygon mirror rotates. Even in the case where the polygon motor isrotated at several patterns of rotation speeds in order to deal withdifferent printing speeds, change in the force applied in the axialdirection can be restrained. Thus, the problem of variance in theposition in the direction of rotation axis of the polygon mirror andhence change in optical characteristics can be avoided.

Meanwhile, in FIG. 4, as θ₂=−θ₄ is set and the thrust-direction forcegenerated by air resistance received by the reflection surfaces d and his canceled by the thrust-direction force generated by air resistancereceived by the reflection surfaces b and f and the reflection surfacesd and h, a deviated thrust-direction force is prevented from beingapplied to the polygon mirror 35 and positional variance in thedirection of rotation axis accompanied by the rotation of the polygonmirror can be restrained. That is, it is possible to stably hold theposition in the direction of rotation axis of the polygon mirror withoutusing any expensive or complicated supporting mechanism and irrespectiveof the rotating movement of the polygon mirror.

Also, with the configuration according to this embodiment, the opticalcharacteristics of the light scanning device can be stabilized, and itis not necessary to take a large effective reflection area of thereflection surfaces in the direction of rotation axis of the polygonmirror in anticipation of change in the position of the polygon mirrorin the axial direction. The thickness of the polygon mirror in thedirection of rotation axis can be reduced. Therefore, windage loss whenthe polygon mirror rotates can be restrained and increase in motor loadand noise can be restrained.

Also, at the time of starting or stopping the rotation, change in theforce applied in the axial direction can be restrained. Therefore, it isnot necessary to increase rigidity of the supporting mechanism byproviding a bearing to support the force in the axial direction or byusing a magnetic force. This can contribute to reduction in the cost ofthe apparatus.

Also, in this embodiment, the reflection surface a and the reflectionsurface e having the angle θ₁ which has the maximum difference in angleto the rotation axis, and the reflection surface c and the reflectionsurface g having the angle θ₃ are not arranged next to each other. Whenthe polygon mirror rotates, the flow of wind in the axial direction isprevented from largely changing, and thus windage loss and noise areprevented. That is, the plural reflection surfaces a to h are arrayed insuch combinations that the difference in the tilt angle with respect tothe rotation axis 35 p between the neighboring reflection surfaces issmaller than the maximum difference in angle (θ₁−θ₃) that can begenerated by a combination of the plural reflection surfaces a to h.

The polygon mirror 35 as described above is arranged on the polygonmotor 36 including a DC brushless motor or the like and is fixed, with aleaf spring, wave washer or the like, onto the bearing surfaceintegrally formed with the rotor of the DC brushless motor.

FIG. 5 is a plan view showing an optical path in the light scanningdevice according to the first embodiment of the invention. FIG. 6 is alongitudinal sectional view showing the optical path in the lightscanning device according to the first embodiment of the invention, asenlarged in the sub scanning direction. FIG. 7 is a view showing anexemplary coordinate system for defining the shape of a lens surface.FIG. 8 is a view showing an exemplary shape defining formula for thelens surface. FIG. 9 is a table showing optical layout data in the lightscanning device according to the first embodiment of the invention. FIG.10 is a table showing paraxial characteristic data in the light scanningdevice according to the first embodiment of the invention. FIG. 11 is atable showing decentration and tilt data in the light scanning deviceaccording to the first embodiment of the invention. FIG. 12 is a tableshowing coefficient data representing the lens shape and so on in thelight scanning device according to the first embodiment of theinvention.

FIG. 9 to FIG. 12 show properties and specifications of the lightscanning device according to this embodiment.

In FIG. 9, REFRACTIVE INDEX AFTER REFLECTION, DISTANCE BETWEEN SURFACESAND SHAPE DATA HAVE NEGATIVE SIGN (−) ADDED THERETO; LENS SURFACE SHAPEWITH CURVED SURFACE POLYNOMIAL COEFFICIENT DATA IS EXPRESSED BY THEFOLLOWING EQUATION:X=(cuy*y²+cuz*z²)/(1+Sqrt(1−ay*cuy²*y²−az*cuz²*z²))=Σa_(lm)*y¹*z^(m); INTHIS EXAMPLE, ay=1, az=1; y DIRECTION: MAIN SCANNING DIRECTION, zDIRECTION: SUB SCANNING DIRECTION, x DIRECTION: DIRECTION OF OPTICALAXIS (+ SIDE IN EACH LOCAL COORDINATE SYSTEM OF PRE-DEFLECTION OPTICALSYSTEM AND − SIDE IN EACH LOCAL COORDINATE SYSTEM OF POST-DEFLECTIONOPTICAL SYSTEM.

The shape of the lens surface of each optical element in the lightscanning device according to this embodiment is expressed, for example,by a shape defining formula as shown in FIG. 8 in the case where theshape of the lens surface is expressed according to a coordinate systemas shown in FIG. 7. In this embodiment, ay=1 and az=1 hold in thedefining formula shown in FIG. 8. O_(R) in FIG. 7 represents the opticalaxis.

FIG. 9 shows that the diameter of the inscribed circle of the polygonmirror 35 is 40.0 mm in this embodiment. FIG. 9 also shows that when themain scanning direction is expressed as Y-direction, the sub scanningdirection as Z-direction and the direction of the optical axis asX-direction (+ before deflection and − after deflection), the positionof the center of rotation of the polygon mirror 35 is at a position thatis 17.5 mm in the X-direction (the direction in which light on theoptical axis of the pre-deflection optical system travels) and 9.8 mm inthe Y-direction (the direction perpendicular to the direction in whichthe light on the optical axis of the pre-deflection optical systemtravels and to the direction of the rotation axis of the polygonmirror), as expressed in the local coordinate system of the reflectionsurface of the polygon mirror 35.

FIG. 9 also shows the tilt angles θ₁ to θ₄ from the center of rotationof the polygon mirror. It can be seen from FIG. 9 that the tilt anglesof the reflection surfaces of the polygon mirror are set at ±θ₁ and ±θ₂.Moreover, the distance to the image forming position of a finite focuslens 72 is set at 814.3 mm from the position of the photoconductor-sideprincipal point of the finite lens.

FIG. 9 also shows the curvature and spacing (TH) on the optical axis oneach surface (incident surface and exit surface) of each opticalelement, the refractive index of each optical element and so on. In FIG.9, the distance (TH) between optical elements that are next to eachother is described as being common to laser beams LY (described as RAY1in FIG. 9), LM (described as RAY2 in FIG. 9), LC (described as RAY4 inFIG. 9) and LK (described as RAY3 in FIG. 9), and also described asbeing different among the laser beams LY (RAY1), LM (RAY2), LC (RAY4)and LK (RAY3).

FIG. 10 is a view showing the paraxial power of the fθ1 lens 37A and thefθ2 lenses 37Y, 37M, 37C and 37K in the first embodiment.

FIG. 11 is a view showing decentration and tilt angle in the localcoordinate system of each optical element. Each optical element in thisembodiment is arranged with the decentration and tilt shown in FIG. 11.Here, “Surface No” used in FIG. 5, FIG. 6, FIG. 9 and FIG. 11 is commonto these drawings.

The “Surface No” in FIG. 5, FIG. 6, FIG. 9 and FIG. 11 represents theorder according to which the light emitted from the LD array 71 passesthrough the surface of each optical element. Here, the surface No1represents the position of the image-side principal point of the finitelens 72. The surface No2 represents the incident surface (toward the LDarray 71) of the cylindrical lens 74A.

In the column corresponding to the surface No1 in FIG. 9, the distanceTN is 33.3 and the refractive index N is 1. This means that lightexiting from the position of the photoconductor-side principal point ofthe finite lens 72 is propagated 33.3 mm through a medium having arefractive index of 1 (through air) and reaches the surface of thesurface No2.

It can be understood from the column corresponding to the surface No2shown in FIG. 9 that the surface of the surface No2 has a curvature0.02078 in the sub scanning direction, that the propagation distance tothe surface of the surface No3 is 5.0 mm, and that this medium has arefractive index of 1.511 (that is, this medium is made of glass).Specifically, the surface No2 represents the incident surface side ofthe cylindrical lens 74A. The cylindrical lens 74A is made of glass andhas a thickness of 5.0 mm, and its exit surface is formed by a planesurface with no curvature.

As the luminous flux exiting from the surface of the surface No3 ispropagated 62.7 mm through air, the luminous flux becomes incident onthe incident surface of the cylindrical lens 74B (surface No4). As theluminous flux incident on the cylindrical lens 74B is propagated throughthe glass having a thickness of 5.0 mm and a refractive index of 1.511,the luminous flux reaches the exit surface having a curvature of 0.08957in the sub scanning direction (surface No5). As the luminous fluxexiting from the surface of the surface No5 is propagated 24.0 mmthrough air with a refractive index of 1, the luminous flux reaches theincident surface of the polygon cover glass, not shown (surface No6). Itcan be seen from FIG. 9 that this polygon cover glass is made of glasswith a refractive index of 1.511 and a thickness of 1.9 mm.

As the luminous flux passed through the polygon cover glass ispropagated 20 mm, the luminous flux reaches the reflection surface ofthe polygon mirror 35, which is a deflection surface.

Next, the propagation via the surface of the surface No9 and thesubsequent surfaces will be described with reference to FIG. 5, FIG. 6,FIG. 9 and FIG. 12.

As the luminous fluxes reflected and deflected by the reflectionsurfaces 35Y, 35M, 35C and 35K of the polygon mirror 35, which aredeflection surfaces, are propagated 6.8 mm through air, the luminousfluxes reach the polygon cover glass made of glass with a thickness of1.9 mm. The surface No10 represents the incident surface of the polygoncover glass. The surface No11 represents its exit surface. As theluminous fluxes passed through the polygon cover glass are propagated25.1 mm through air, the luminous fluxes reach the fθ1 lens 37A. Thesurface No12 represents the incident surface of the fθ1 lens 37A. Thesurface No13 represents the exit surface of the fθ1 lens 37A. As shownin FIG. 9, it can be understood that this fθ1 lens 37A is made ofplastics with a refractive index of 1.503 and with thickness of 9.8 mm.Here, the lens surface shape coefficient of the incident surface of thefθ1 lens 37A is shown in the coefficient table 1 of FIG. 12. The lenssurface shape coefficient of the exit surface of the fθ1 lens 37A isshown in the coefficient table 2 of FIG. 12.

The luminous fluxes guided as described above are propagated through airby a distance (275.5 mm, 275.4 mm, 275.1 mm and 274.6 mm) correspondingto each photoconductive drum (21B, 22B, 23B and 24B), and then reach thefθ2 lenses 37Y, 37M, 37C and 37K. The surface No14 represents theincident surface of the fθ2 lenses. The surface No15 represents the exitsurface of the fθ2 lenses. It can be understood from FIG. 9 that the fθ2lenses are made of plastics with a refractive index of 1.503 and with athickness of 4.5 mm.

Here, the lens surface shape coefficient of the incident surface of thefθ2 lenses is shown in the coefficient table 3 of FIG. 12. The lenssurface shape coefficient of the exit surface of the fθ2 lenses 37Y,37M, 37C and 37K is shown in the coefficient table 4 of FIG. 12.

Then, as the luminous fluxes which have been passed through the fθ2lenses are propagated 27.2 mm through air, the luminous fluxes reach thecover glasses (38Y, 38M, 38C and 38K) having a thickness of 1.9 mm. Theincident surface of the cover glasses is represented by the surfaceNo17. Its exit surface is represented by the surface No18.

Then, the luminous fluxes which have been passed through the coverglasses are propagated by a predetermined distance through air (38 mmfor RAY1, 38 mm for RAY2, 38.3 mm for RAY3 and 38.6 mm for RAY4), andreach the surface of each photoconductive drum.

FIG. 6 shows a longitudinal sectional view of the light scanning deviceaccording to the first embodiment of the invention, as enlarged in thesub scanning direction. More specifically, in the state where the coverglasses toward the photoconductive surfaces of the photoconductors(image surface) are eliminated and the folding of the optical path bythe folding mirrors is carried out, the optical path at the center ofthe tilting angle is indicated by a solid line, and rays with maximumand minimum tilting angles are indicated by a dotted line and adouble-chain-dotted line. In FIG. 6, the geometric figure of the lens onthe side of the photoconductive surface represents the position of theoptical axis of the lens on the side of the photoconductive surface. Thebold double-chain-dotted line on the left side indicates the focalposition toward the object point side in the sub scanning direction ofthe fθ lens 37A. In the post-deflection optical system, all the rayspass through the single fθ1 lens 37A, and the rays corresponding to thetilt angles on the individual polygon mirror reflection surfaces (θ₃ forthe laser beam LY (RAY1), θ₂ for the laser beam LM (RAY2), θ₁ for thelaser beam LC (RAY4), and θ₄ for the laser beam LK (RAY3)) pass throughthe fθ2 lenses 37Y, 37M, 37C and 37K.

As can be seen from FIG. 6, the spacing between the rays situated atboth ends in the sub scanning direction (vertical direction) (rays LYand LC corresponding to θ₁ and θ₃, which have the maximum absolute valueof tilt angle from the central axis of rotation of the polygon mirror)and the rays next to them is greater than the spacing between the innerrays (rays LM and LK corresponding to θ₂ and θ₄, which do not have themaximum absolute value of tilt angle from the central axis of rotationof the polygon mirror).

Since the distance between rays becomes greater toward the downstream ofthe rays, the rays having a large spacing from the next beam (rays LYand LC corresponding to θ₁ and θ₃, which have the maximum absolute valueof tilt angle from the central axis of rotation of the polygon mirror)can be separated from the other rays on the upstream side of the rays.

Also, in FIG. 3, the ray LC and the ray LY are the rays situated at bothends in the sub scanning direction. The ray LY is separated from theother rays at the most upstream part. The ray LC is separated from theother rays at the third most upstream part. In this way, as shown inFIG. 20, which will be described later, the second folding mirror 34Cfor the ray LC, and the ray LK passing through the outermost side can beprevented from interfering with each other, with respect to the casewhere one of the rays situated at both ends in the sub scanningdirection is the ray LK.

Therefore, as the scanning optical system providing four scanning linesemploys such a configuration that the position where the rays situatedat both ends in the sub scanning direction are separated in thepost-deflection optical system is the most upstream part (polygon mirrorside) of the optical path and the third most upstream part, the mountingspace can be reduced. Moreover, it is not necessary to carry outchamfering processing to the folding mirror in order to avoidinterference between the ray and the folding mirror.

FIG. 13 is a graph showing the relation between stray light and theposition of a ray in the main scanning direction in the light scanningdevice according to the first embodiment of the invention.

FIG. 13 shows a graph in which the main scanning direction of a writingray is plotted on the horizontal axis, and in which the position in themain scanning direction when stray light reflected on the exit surfaceof the fθ2 lens is reflected again on the incident surface of the fθ2lens and reaches the photoconductive surface, is plotted on the verticalaxis. The dotted line indicates the boundary of an effective area.

It can be seen from FIG. 13 that: (1) in the effective area, the straylight monotonously increases as the position of the writing ray shiftsto the + (positive) side; and (2) its maximum and minimum values areoutside of the effective area.

It can be seen from the above description (1) that the position of thestray light moves along with the movement of the writing light. It canbe understood that the configuration according to this embodiment iseffective for preventing accumulation of energy and its appearance onthe image which would be caused by the immobile stray light when thewriting ray is moving, since the stray light generated within the lensthat is closest to the photoconductive surface is generally difficult tointerrupt by a light shielding member.

Also, it can be seen from the above description (2) that the stray lightis situated on the outer side in the effective area than the writingray. Moreover, it can be understood that when the writing light hasreached both ends of the effective area on the photoconductive drum, thestray light is situated outside of the effective area in the mainscanning direction.

Second Embodiment

Next, the second embodiment of the invention will be described. Thisembodiment is a modification of the first embodiment. Hereinafter, inthis embodiment, the units and elements having the similar functions asthose described in the first embodiment are denoted by the samereference numerals and will not be described further in detail.

In the second embodiment, compared to the first embodiment 1, whileθ₁=−θ₃ holds for the surfaces having the largest absolute value of tiltangle in the polygon mirror 35 with respect to the rotation axis, θ₂≠−θ₄is given. The lens system in the light scanning device according to thisembodiment has a configuration equivalent to the configuration of thefirst embodiment.

FIG. 14 is a plan view showing an optical path in a light scanningdevice according to the second embodiment of the invention. FIG. 15 is alongitudinal sectional view showing the optical path in the lightscanning device according to the second embodiment of the invention, asenlarged in the sub scanning direction. FIG. 16 is a table showingoptical layout data in the light scanning device according to the secondembodiment of the invention. FIG. 17 is a table showing paraxialcharacteristic data in the light scanning device according to the secondembodiment of the invention. FIG. 18 is a table showing decentration andtilt data in the light scanning device according to the secondembodiment of the invention. FIG. 19 is a table showing coefficient datarepresenting the lens shape in the light scanning device according tothe second embodiment of the invention.

Here, FIG. 16 corresponds to FIG. 9 in the first embodiment. FIG. 17corresponds to FIG. 10 in the first embodiment. FIG. 18 corresponds toFIG. 11 in the first embodiment. FIG. 19 corresponds to FIG. 12 in thefirst embodiment. Moreover, FIG. 14 corresponds to FIG. 5 in the firstembodiment. FIG. 15 corresponds to FIG. 6 in the first embodiment.

In FIG. 16, REFRACTIVE INDEX AFTER REFLECTION, DISTANCE BETWEEN SURFACESAND SHAPE DATA HAVE NEGATIVE SIGN (−) ADDED THERETO; LENS SURFACE SHAPEWITH CURVED SURFACE POLYNOMIAL COEFFICIENT DATA IS EXPRESSED BY THEFOLLOWING EQUATION:X=(cuy*y²+cuz*z²)/(1+Sqrt(1−ay*cuy²*y²−az*cuz²*z²))=Σa_(lm)*y¹*z^(m); INTHIS EXAMPLE, ay=1, az=1; y DIRECTION: MAIN SCANNING DIRECTION, zDIRECTION: SUB SCANNING DIRECTION, x DIRECTION: DIRECTION OF OPTICALAXIS (+ SIDE IN EACH LOCAL COORDINATE SYSTEM OF PRE-DEFLECTION OPTICALSYSTEM AND − SIDE IN EACH LOCAL COORDINATE SYSTEM OF POST-DEFLECTIONOPTICAL SYSTEM.

In FIG. 15, the optical path at the center of the tilting angle isindicated by a solid line, and rays with maximum and minimum tiltingangles are indicated by a dotted line and a double-chain-dotted line.The geometric figure of the lens on the side of the photoconductivesurface represents the position of the optical axis in FIG. 15. The bolddouble-chain-dotted line on the left side indicates the focal positiontoward the object point side in the sub scanning direction of the fθ1lens 37A.

As can be seen from FIG. 15, the spacing between the rays next to eachother is greater for the rays closer to the top side in FIG. 15.

FIG. 20 is a plan view of the optical path of a ray cast for scanning bythe light scanning device according to the second embodiment of theinvention, as viewed from above. FIG. 21 is a side view of the opticalpath of the ray cast for scanning in an image forming apparatus havingthe light scanning device according to the second embodiment of theinvention, as viewed from a lateral side.

FIG. 21 shows a configuration in which rays are guided to fourphotoconductive drums by using folding mirrors. In this embodiment, therays from the top side in FIG. 21 are separated in order on the upstreamside. That is, the rays reflected and deflected by the polygon mirror 35are separated in order of the ray LY, the ray LM, the ray LC and the rayLK and reach the first folding mirrors 33Y, 33M, 33C and 33K. Then, therays LY, LM, LC and LK reach the second folding mirrors 34Y, 34M, 34Cand 34K, pass through the fθ2 lenses 37Y, 37M, 37C and 37K correspondingto the individual photoconductive drums, pass through the cover glasses38Y, 38M, 38C and 38K, and are then guided to the photoconductive drums.

In this configuration, the side of the second folding mirror for the rayLC that is closer to the ray LK is chamfered in order to preventinterference between the second folding mirror for the ray LC and theray LK passing the outermost side.

FIG. 22 is a graph showing the relation between stray light and theposition of the scanning light in the main scanning direction in thelight scanning device according to the second embodiment of theinvention.

FIG. 22 shows a graph in which the main scanning direction of a writingray is plotted on the horizontal axis, and the position in the mainscanning direction when stray light reflected on the exit surface of thefθ2 lens is reflected again on the incident surface of the fθ2 lens andreaches the image surface, is plotted on the vertical axis. The dottedline indicates the boundary of an effective area.

Similarly to FIG. 13 shown in the first embodiment, it can be seen fromFIG. 22 that: (1) in the effective area, the stray light monotonouslyincreases as the position of the writing ray shifts toward the +(positive) side; and (2) its maximum and minimum values are outside ofthe effective area.

It can be seen from the above description (1) that the position of thestray light moves along with the movement of the writing light. It canbe understood that the configuration of this embodiment is effective forpreventing accumulation of energy and its appearance on the image whichwould be caused by the immobile stray light when the writing ray ismoving, since the stray light generated within the lens that is closestto the photoconductive surface is generally difficult to interrupt by alight shielding member.

Also, it can be seen from the above description (2) that the stray lightis situated on the outer side in the effective area than the writingray. Moreover, it can be understood that that when the writing light hasreached both ends of the effective area on the photoconductive drum, thestray light is situated outside of the effective area in the mainscanning direction.

To realize the above functions in the light scanning device according tothis embodiment, the lens that is closest to the photoconductive sidehas such a shape that its thickness increases toward the edge in themain scanning direction.

Third Embodiment

Next, the third embodiment of the invention will be described. Thisembodiment is a modification of the above first embodiment. Hereinafter,in this embodiment, the units and elements having the similar functionsas those described in the first embodiment are denoted by the samereference numerals and will not be described further in detail.

In the third embodiment, the polygon mirror 35 and the scanning opticalsystem which have the similar configuration as in the first embodimentare used, and a diffraction element surface is appended to the exit sideof the four fθ2 lenses 37Y, 37M, 37C and 37K, thus optimizing opticalcharacteristics.

FIG. 23 is a plan view showing an optical path in a light scanningdevice according to the third embodiment of the invention. FIG. 24 is alongitudinal sectional view showing the optical path in the lightscanning device according to the third embodiment of the invention, asenlarged in the sub scanning direction. FIG. 25 is a table showingoptical layout data in the light scanning device according to the thirdembodiment of the invention. FIG. 26 is a table showing paraxialcharacteristic data in the light scanning device according to the thirdembodiment of the invention. FIG. 27 is a table showing decentration andtilt data in the light scanning device according to the third embodimentof the invention. FIG. 28 is a table showing coefficient datarepresenting the lens shape in the light scanning device according tothe third embodiment of the invention.

Here, FIG. 25 corresponds to FIG. 9 in the first embodiment. FIG. 26corresponds to FIG. 10 in the first embodiment. FIG. 27 corresponds toFIG. 11 in the first embodiment. FIG. 28 corresponds to FIG. 12 in thefirst embodiment. Moreover, FIG. 23 corresponds to FIG. 5 in the firstembodiment. FIG. 24 corresponds to FIG. 6 in the first embodiment.

In FIG. 25, REFRACTIVE INDEX AFTER REFLECTION, DISTANCE BETWEEN SURFACESAND SHAPE DATA HAVE NEGATIVE SIGN (−) ADDED THERETO; LENS SURFACE SHAPEWITH CURVED SURFACE POLYNOMIAL COEFFICIENT DATA IS EXPRESSED BY THEFOLLOWING EQUATION:X=(cuy*y²+cuz*z²)/(1+Sqrt(1−ay*cuy²*y²−az*cuz²*z²))=Σa_(lm)*y¹*z^(m); INTHIS EXAMPLE, ay=1, az=1; y DIRECTION: MAIN SCANNING DIRECTION, zDIRECTION: SUB SCANNING DIRECTION, x DIRECTION: DIRECTION OF OPTICALAXIS (+ SIDE IN EACH LOCAL COORDINATE SYSTEM OF PRE-DEFLECTION OPTICALSYSTEM AND − SIDE IN EACH LOCAL COORDINATE SYSTEM OF POST-DEFLECTIONOPTICAL SYSTEM.

The configuration shown in FIG. 23 is substantially the same as theconfiguration shown in FIG. 5 in the first embodiment. However, thethird embodiment is different from the first embodiment in that adiffraction element surface is provided on the exit side of the fθ2lenses 37Y, 37M, 37C and 37K.

As the diffraction element surface is provided as described above, theoptical coefficient of these fθ2 lenses 37Y, 37M, 37C and 37K changes.The various optical coefficients and characteristics in the lightscanning device as a whole according to the third embodiment take valuesas shown in FIG. 26 to FIG. 28.

As can be seen from FIG. 24, the spacing between the rays situated atboth ends in the sub scanning direction (vertical direction) (rayscorresponding to θ₁ and θ₃, which have the maximum absolute value oftilt angle from the central axis of rotation of the polygon mirror) andthe rays next to them is greater than the spacing between the rayscorresponding to θ₂ and θ₄, which do not have the maximum absolute valueof tilt angle from the central axis of rotation of the polygon mirror.This feature is similar to the first embodiment.

Also, since the distance between rays becomes greater toward thedownstream in the traveling direction of the rays, the rayscorresponding to θ₁ and θ₃, which have the maximum absolute value oftilt angle from the central axis of rotation of the polygon mirror, canbe separated from the other rays on the upstream side of the rays.

Therefore, even in the case where the diffraction element surface isprovided on the exit surfaces of the fθ2 lenses 37Y, 37M, 37C and 37K,the rays can be guided to the photoconductive drums through the opticalpath similar to the optical path shown in FIG. 2 and FIG. 3 in the firstembodiment.

That is, the rays LC and LY are rays situated at both ends in the subscanning direction. The ray LY is separated from the other rays at themost upstream position and the ray LC is separated from the other raysat the third most upstream position. In this way, the second foldingmirror 34C for the ray LC, and the ray LK passing through the outermostside in the sub scanning direction can be prevented from interferingwith each other, compared to the configuration as shown in FIG. 21 inwhich the ray LK is set as a ray at one end.

Thus, even in the configuration in which the diffraction element surfaceis provided on the exit surfaces of the fθ2 lenses, as in thisembodiment, as the configuration is employed in which, in the scanningoptical system providing four scanning lines, the position where therays at both ends in the sub scanning direction are separated in thepost-deflection optical system is the most upstream part (polygon mirrorside) of the optical path and the third most upstream part, the mountingspace can be reduced. Moreover, it is not necessary to carry outchamfering processing to the folding mirror in order to avoidinterference between the rays and the folding mirror.

The function of optical path difference of the diffraction gratingsurface is expressed by ΣC_(LM)×Y^(L)×z^(M).

The fθ2 lenses in this case are plastic lenses on which individualluminous fluxes reflected and deflected by the polygon mirror 35 becomeincident at different incident positions from each other in the subscanning direction orthogonal to the main scanning direction, of theplural optical elements forming the post-deflection optical system(irrespective of whether an individual optical element is used for eachluminous flux or a common optical element is used for all the luminousfluxes). It suffices that the luminous fluxes incident on the fθ2 lensesare incident at different incident positions from each other in the subscanning direction. One of the plural luminous fluxes may becomeincident through the optical axis of the post-deflection optical system.In this way, as the diffraction grating is formed on the opticalelements on which the luminous fluxes from the respective lights sourcesbecome incident at different positions from each other in the subscanning direction, relative space adjustment and angle adjustmentbetween the luminous fluxes can be made according to temperature change.

The optical element on which the diffraction grating should be formedmay be an optical element on which a luminous flux from the light sourcebecomes incident at a different incident position from the optical pathof the optical axis of the post-deflection optical system in the subscanning direction orthogonal to the main scanning direction, of theplural optical elements forming the post-deflection optical system.Basically, the function of curving the optical path cannot be providedfor a luminous flux which becomes incident through the optical axis.Therefore, in order to correct chromatic aberration according totemperature change by the diffraction grating, it is desired that theluminous flux should be made incident at a position that is at leastdifferent from the optical axis.

Also, the diffraction grating formed on the exit surface of the fθ2 lenshas power in the sub scanning direction. Thus, occurrence of“longitudinal chromatic aberration” and “traverse chromatic aberration”can be restrained. Here, “traverse chromatic aberration” is equivalentto “chromatic aberration of magnification”. “Longitudinal chromaticaberration” is equivalent to chromatic aberration which occurs in thedirection of optical axis (that is, the focal point or the position ofthe image point on the axis varies according to wavelength).

The diffraction grating formed on the exit surface of the fθ2 lens doesnot necessarily have to have power in the sub scanning direction and mayhave power in the main scanning direction alone. Thus, in the case wherethe diffraction grating formed on the fθ2 lens is a diffraction gratinghaving power only in the main scanning direction, occurrence of“longitudinal chromatic aberration” can be restrained (that is,defocusing variation when the temperature has changed can be reduced).Further, it is possible to restrain fluctuation of f of the fθcharacteristics due to the wavelength fluctuation.

Of course, in consideration of the manufacturing cost and the number ofprocess steps, the diffraction grating formed on the fθ2 lens may havepower both in the main scanning direction and in the sub scanningdirection.

Also, as the optical element having the diffraction grating surfaceformed thereon is the fθ2 lens having a curved incident surface and exitsurface, wavefront aberration on the image surface can be improved whilethe beam position and defocusing are corrected according to temperaturechange (temperature compensation).

Here, in this example, the diffraction grating surface is formed on theexit surface of each fθ2 lens provided for each photoconductor. However,the configuration is not limited to this. As a matter of course, adiffraction element surface can be similarly provided on an fθ2 lensshared by plural photoconductors (shared optical element). Also, in thisexample, the diffraction grating surface is formed on the fθ2 lenses.However, the configuration is not limited to this. For example, it isalso possible to form a diffraction grating surface on an arbitrarysurface of an arbitrary optical element which satisfies the aboveconditions, of the plural optical elements forming the light scanningdevice, such as the fθ1 lens.

Fourth Embodiment

Next, the fourth embodiment of the invention will be described. Thisembodiment is a modification of the above first embodiment. Hereinafter,in this embodiment, the units and elements having the similar functionsas those described in the first embodiment are denoted by the samereference numerals and will not be described further in detail.

In the fourth embodiment, a diffraction element surface is appended tothe exit side of the fθ1 lens in the light scanning device according tothe first embodiment, thus optimizing optical characteristics.

FIG. 29 is a plan view showing an optical path in a light scanningdevice according to the fourth embodiment of the invention. FIG. 30 is alongitudinal sectional view showing the optical path in the lightscanning device according to the fourth embodiment of the invention, asenlarged in the sub scanning direction. FIG. 31 is a table showingoptical layout data in the light scanning device according to the fourthembodiment of the invention. FIG. 32 is a table showing paraxialcharacteristic data in the light scanning device according to the fourthembodiment of the invention. FIG. 33 is a table showing decentration andtilt data in the light scanning device according to the fourthembodiment of the invention. FIG. 34 is a table showing coefficient datarepresenting the lens shape in the light scanning device according tothe fourth embodiment of the invention.

Here, FIG. 31 corresponds to FIG. 9 in the first embodiment. FIG. 32corresponds to FIG. 10 in the first embodiment. FIG. 33 corresponds toFIG. 11 in the first embodiment. FIG. 34 corresponds to FIG. 12 in thefirst embodiment. Moreover, FIG. 29 corresponds to FIG. 5 in the firstembodiment. FIG. 30 corresponds to FIG. 6 in the first embodiment.

In FIG. 31, REFRACTIVE INDEX AFTER REFLECTION, DISTANCE BETWEEN SURFACESAND SHAPE DATA HAVE NEGATIVE SIGN (−) ADDED THERETO; LENS SURFACE SHAPEWITH CURVED SURFACE POLYNOMIAL COEFFICIENT DATA IS EXPRESSED BY THEFOLLOWING EQUATION:X=(cuy*y²+cuz*z²)/(1+Sqrt(1−ay*cuy²*y²−az*cuz²*z²))=Σa_(lm)*y¹*z^(m); INTHIS EXAMPLE, ay=1, az=1; y DIRECTION: MAIN SCANNING DIRECTION, zDIRECTION: SUB SCANNING DIRECTION, x DIRECTION: DIRECTION OF OPTICALAXIS (+ SIDE IN EACH LOCAL COORDINATE SYSTEM OF PRE-DEFLECTION OPTICALSYSTEM AND − SIDE IN EACH LOCAL COORDINATE SYSTEM OF POST-DEFLECTIONOPTICAL SYSTEM.

The configuration shown in FIG. 29 is substantially the same as theconfiguration shown in FIG. 5 in the first embodiment. However, thefourth embodiment is different from the first embodiment in that adiffraction element surface is provided on the exit side of the fθ1lens.

Since the diffraction element is provided as in this embodiment, theoptical coefficients of the fθ2 lenses 37Y, 37M, 37C and 37K change. Asa whole, various optical coefficients and characteristics in the lightscanning device according to the fourth embodiment take values as shownin FIG. 31 to FIG. 34.

As can be seen from FIG. 30, the spacing between the rays situated atboth ends in the sub scanning direction (vertical direction) (rayscorresponding to θ₁ and θ₃, which have the maximum absolute value oftilt angle from the central axis of rotation of the polygon mirror) andthe rays next to them is greater than the spacing between the rayscorresponding to θ₂ and θ₄, which do not have the maximum absolute valueof tilt angle from the central axis of rotation of the polygon mirror.This feature is similar to the first embodiment.

Also, since the distance between rays becomes greater toward thedownstream in the traveling direction of the rays, the rayscorresponding to θ₁ and θ₃, which have the maximum absolute value oftilt angle from the central axis of rotation of the polygon mirror, canbe separated from the other rays on the upstream side of the rays.

Therefore, even in the case where the diffraction element surface isprovided on the exit surface of the fθ1 lens 37A, the rays can be guidedto the photoconductive drums through the optical path similar to theoptical path shown in FIG. 2 and FIG. 3 of the first embodiment.

That is, the rays LC and LY are rays situated at both ends in the subscanning direction. The ray LY is separated from the other rays at themost upstream position and the ray LC is separated from the other raysat the third most upstream position. In this way, the second foldingmirror 34C for the ray LC, and the ray LK passing through the outermostside in the sub scanning direction can be prevented from interferingwith each other, compared to the case where the ray LK is set as a rayat one end.

Thus, even in the case where the diffraction element surface is providedon the exit surface of the fθ1 lens 37A, as in this embodiment, as theconfiguration is employed in which the position where the rays situatedat both ends in the sub scanning direction are separated in thepost-deflection optical system is the most upstream part (polygon mirrorside) of the optical path and the third most upstream part, the mountingspace can be reduced. Moreover, it is not necessary to carry outchamfering processing to the folding mirror in order to avoidinterference between the rays and the folding mirror.

In any of the above embodiments, as shown in FIG. 9 to FIG. 28, thepost-deflection optical system has a shared optical element which passesplural rays to be guided to plural photoconductors and has positivepower in the main scanning direction and the sub scanning direction, andin which the power in the sub scanning direction is greater, and a lenswhich has positive power in the sub scanning direction acting onluminous fluxes reflected and deflected by plural reflection surfaces ofthe polygon mirror. In such a post-deflection optical system, thecurvature in the main scanning direction and the sub scanning directionof both lenses forming the post-deflection optical system is changedseparately in accordance with the position in the main scanningdirection and the position in the sub scanning direction. Thus, the fθcharacteristic, face tangle error correction characteristic and imageforming characteristic that are necessary for the scanning opticalsystem are realized.

In the above embodiments, a single optical element is used as theoptical element that passes all the rays in the post-deflection opticalsystem. However, the configuration is not limited to this. For example,in the case where this optical element may include plural lenses and theplural lenses are provided with power, it suffices that the combinedpower in the entire optical system holds the above-described relation.In this case, it is possible to employ a configuration in which thecurvature is not changed according to the position in the main scanningand sub scanning directions.

Also, in the case where light reflected and deflected by the individualreflection surfaces of the polygon mirror is made incident on a lenswhich has positive power in the sub scanning direction acting on therays corresponding to the reflection surfaces, if the lens is providedwith negative power in the main scanning direction at the central partof the lens, the thickness at the central part of the lens can beprevented from increasing. Also, the power is increased toward the edgeof the lens, thereby realizing performance of the fθ characteristic andimage forming characteristic.

The lens which has positive power in the sub scanning direction actingon the rays reflected and deflected by the reflection surfaces havingdifferent tilt angles from each other with respect to the rotation axisof the polygon mirror has negative power in the main scanning directionat the central part of the lens. Thus, the thickness at the central partof the lens can be prevented from increasing.

In the configuration described in the above embodiments, at least a partof the first thrust-direction force generated by air resistance receivedby at least one surface of the reflection surfaces tilted with respectto the rotation axis of the polygon mirror is canceled by the secondthrust-direction force generated by air resistance received by a surfacetilted in the opposite direction to the surface that generates the firstthrust-direction force with respect to the rotation axis (it may also bethe reduction of the second thrust-direction force by the firstthrust-direction force). However, the configuration is not limited tothis. For example, a propeller which generates the secondthrust-direction force as the polygon mirror rotates may be provided atan end part of the rotation axis of the polygon mirror.

Also, according to one embodiment of the invention, a light scanningdevice can be provided which includes: a rotary deflector configured toreflect and deflect an incident luminous flux by plural reflectionsurfaces arrayed in a rotating direction and thus cause the incidentluminous flux to scan a predetermined direction, in which at least oneof the plural reflection surfaces is tilted with respect to the rotationaxis of the rotary deflector; and thrust-direction force canceling meanssupported in a rotatable manner integrally with the plural reflectionsurfaces of the rotary deflector, and for generating, by air resistance,a force in a direction opposite to a thrust-direction force generated byair resistance received by at least one surface of the reflectionsurfaces tilted with respect to the rotation axis.

In the light scanning device having the configuration as describedabove, it is desirable that the thrust-direction force canceling meansgenerates a thrust-direction force by air resistance at the time ofrotation of the rotary deflector, on at least one surface of thereflection surfaces tilted with respect to the rotation axis of therotary deflector.

In the light scanning device having the configuration as describedabove, the rotary deflector has plural reflection surfaces withdifferent tilt angles from each other with respect to the rotation axis,and the thrust-direction force canceling means can be configured togenerate a thrust-direction force by air resistance at the time ofrotation of the rotary deflector, on a surface having the largest tiltangle with respect to the rotation axis, of the plural reflectionsurfaces.

In the light scanning device having the configuration as describedabove, it is preferable that the thrust-direction force canceling meansis configured to generate a thrust-direction force by air resistance atthe time of rotation of the rotary deflector, on a surface tilted withrespect to the rotation axis at the same angle as and in the oppositedirection to a reflection surface having the largest tilt angle withrespect to the rotation axis, of the plural reflection surfaces.

In the light scanning device having the configuration as describedabove, the plural reflection surfaces can be configured to be arrayed insuch a combination that the difference in tilt angle with respect to therotation axis between neighboring reflection surfaces is smaller thanthe maximum difference in angle that can be generated by a combinationof the plural reflection surfaces.

In the light scanning device having the configuration as describedabove, it is desirable that the light scanning device causes a luminousflux from a light source to scan a photoconductive surface of each ofplural photoconductors in a main scanning direction, and that the lightscanning device has a pre-deflection optical system which shapes thelight from the light source into a luminous flux having a predeterminedsectional shape, guides the luminous flux toward the rotary deflectorand condenses the luminous flux in a sub scanning direction in thevicinity of the reflection surfaces of the rotary deflector, and apost-deflection optical system which includes plural optical elementsand guides the luminous flux reflected and deflected by each of theplural reflection surfaces of the rotary deflector to thephotoconductive surface of the photoconductor corresponding to eachreflection surface, wherein the post-deflection optical system include ashared optical element which provides, to the luminous flux which isreflected and deflected by the rotary deflector and should be guided toeach of the plural photoconductors, such power that the luminous fluxguided to the photoconductor surface by the post-deflection opticalsystem has a predetermined optical characteristic on the photoconductorsurface in accordance with the incident position of the luminous flux.

In the light scanning device having the configuration as describedabove, it is preferable that the predetermined optical characteristic isat least one of the beam diameter of the luminous flux, the degree ofcurving of a scanning line, and the position of the luminous flux withrespect to a scanning area.

In the light scanning device having the configuration as describedabove, of the plural optical elements forming the post-deflectionoptical system, in at least one optical element on which the luminousflux to be guided to each of the plural photoconductors becomes incidentat different incident positions from each other in a sub scanningdirection orthogonal to the main scanning direction, a diffractiongrating can be formed on at least one of an incident surface and an exitsurface for the luminous flux in the optical element.

In the light scanning device having the configuration as describedabove, it is desirable that, of the plural optical elements forming thepost-deflection optical system, in at least one optical element on whichthe luminous flux from the light source becomes incident at a differentincident position from the optical path of the optical axis of thepost-deflection optical system in a sub scanning direction orthogonal tothe main scanning direction, a diffraction grating is formed on at leastone of an incident surface and an exit surface for the luminous flux inthe optical element.

Also, according to still another embodiment, an image forming apparatuscan be provided which includes: a light scanning device having theconfiguration as described above; a photoconductor on which anelectrostatic latent image is formed by a luminous flux cast forscanning by the light scanning device; and a developing unit configuredto develop the electrostatic latent image formed on the photoconductor.

Moreover, according to still another embodiment, a rotary deflector canbe provided which reflects and deflects an incident luminous flux byplural reflection surfaces arrayed in a rotating direction and thuscauses the incident luminous flux to scan a predetermined direction, andwhich includes: plural reflection surfaces arrayed in a rotatingdirection, with one surface of the plural reflection surfaces beingtilted with respect to a rotation axis; and a thrust-direction forcecanceling unit supported in a rotatable manner integrally with theplural reflection surfaces and having a wind receiving surface tiltedwith respect to the rotation axis of the rotary deflector in a directionopposite to at least one surface of the reflection surfaces tilted withrespect to the rotation axis.

The specific embodiments of the present invention have been described indetail. However, it is obvious to those skilled in the art that variouschanges and modifications can be made without departing from the spiritand scope of the invention.

As described above in detail, according to the present invention, in alight scanning technique of reflecting and deflecting incident light bya rotary deflector, a technique to reduce adverse effects of athrust-direction force generated by air resistance received by areflection surface of the rotary deflector can be provided.

1. A light scanning device comprising: a rotary deflector configured toreflect and deflect an incident luminous flux by plural reflectionsurfaces arrayed in a rotating direction and thus cause the incidentluminous flux to scan a predetermined direction, in which at least oneof the plural reflection surfaces is tilted with respect to the rotationaxis of the rotary deflector; and thrust-direction force cancelingmember supported in a rotatable manner integrally with the pluralreflection surfaces of the rotary deflector, and for generating, by airresistance, a force in a direction opposite to a thrust-direction forcegenerated by air resistance received by at least one surface of thereflection surfaces tilted with respect to the rotation axis.
 2. Thelight scanning device according to claim 1, wherein the thrust-directionforce canceling member generates a thrust-direction force by airresistance at the time of rotation of the rotary deflector, on at leastone surface of the reflection surfaces tilted with respect to therotation axis of the rotary deflector.
 3. The light scanning deviceaccording to claim 2, wherein the rotary deflector has plural reflectionsurfaces with different tilt angles from each other with respect to therotation axis, and the thrust-direction force canceling member generatesa thrust-direction force by air resistance at the time of rotation ofthe rotary deflector, on a surface having the largest tilt angle withrespect to the rotation axis, of the plural reflection surfaces.
 4. Thelight scanning device according to claim 3, wherein the thrust-directionforce canceling member generates a thrust-direction force by airresistance at the time of rotation of the rotary deflector, on a surfacetilted with respect to the rotation axis at the same angle as and in adirection opposite to a reflection surface having the largest tilt anglewith respect to the rotation axis, of the plural reflection surfaces. 5.The light scanning device according to claim 3, wherein the pluralreflection surfaces are arrayed in such a combination that thedifference in tilt angle with respect to the rotation axis betweenneighboring reflection surfaces is smaller than the maximum differencein angle that can be generated by a combination of the plural reflectionsurfaces.
 6. The light scanning device according to claim 1, wherein thelight scanning device causes a luminous flux from a light source to scana photoconductive surface of each of plural photoconductors in a mainscanning direction, the light scanning device has a pre-deflectionoptical system which shapes the light from the light source into aluminous flux having a predetermined sectional shape, guides theluminous flux toward the rotary deflector and condenses the luminousflux in a sub scanning direction in the vicinity of the reflectionsurfaces of the rotary deflector, and a post-deflection optical systemwhich includes plural optical elements and guides the luminous fluxreflected and deflected by each of the plural reflection surfaces of therotary deflector to the photoconductive surface of the photoconductorcorresponding to each reflection surface, wherein the post-deflectionoptical system includes a shared optical element which provides, to theluminous flux which is reflected and deflected by the rotary deflectorand should be guided to each of the plural photoconductors, such powerthat the luminous flux guided to the photoconductor surface by thepost-deflection optical system has a predetermined opticalcharacteristic on the photoconductor surface in accordance with theincident position of the luminous flux.
 7. A thrust-direction forcecanceling method comprising canceling at least a part of a firstthrust-direction force generated by air resistance received by at leastone surface of reflection surfaces arrayed in a rotating direction of arotary deflector and tilted with respect to the rotation axis of therotary deflector, by a second thrust-direction force generated by airresistance received by a surface tilted with respect to the rotationaxis in a direction opposite to the surface where the firstthrust-direction force is generated.
 8. The thrust-direction forcecanceling method according to claim 7, wherein the secondthrust-direction force by air resistance at the time of rotation of therotary deflector is generated on at least one surface of the reflectionsurfaces tilted with respect to the rotation axis of the rotarydeflector.
 9. The thrust-direction force canceling method according toclaim 8, wherein the rotary deflector has plural reflection surfaceshaving different tilt angles from each other with respect to therotation axis, and the second thrust-direction force by air resistanceat the time of rotation of the rotary deflector is generated on asurface having the largest tilt angle with respect to the rotation axis,of the plural reflection surfaces.
 10. The thrust-direction forcecanceling method according to claim 9, wherein the secondthrust-direction force is generated by air resistance at the time ofrotation of the rotary deflector, on a surface tilted in a directionopposite to the surface having the largest title angle with respect tothe rotation axis, of the plural reflection surfaces, and at the sameangle as the reflection surface.
 11. The thrust-direction forcecanceling method according to claim 9, wherein the plural reflectionsurfaces are arrayed in such a combination that difference in tilt anglewith respect to the rotation axis between neighboring reflectionsurfaces is smaller than maximum difference in angle that can begenerated by a combination of the plural reflection surfaces.